1. Field of the Invention
The present invention relates to a method of manufacturing a wide metal thin strip through rapid cooling of a molten metal on the surface of a cooling roll which rotates at high speeds, causing solidification of the molten metal.
2. Description of the Related Art
Various methods have been proposed for manufacturing a metal thin strip (hereinafter "thin strip") from molten metal. Particularly for the manufacture of a wide thin strip, a method known as the planar-flow-casting process is believed to be advantageous. More specifically, as showing in FIG. 1, this method comprises arranging a nozzle having a slit-shaped opening, provided on the bottom of a molten metal vessel 2 in the vicinity of a cooling roll 1 which rotates at high speeds. Molten metal is ejected from an opening 3 of the nozzle onto the surface of the cooling roll 1. The molten metal rapidly cools on the cooling roll, causing it to solidify and form a thin strip 4. Important operational factors in this technique include the distance between the nozzle and the roll, the peripheral speed of the roll, the ejecting pressure of the molten metal and the gap of the slit-shaped opening of the nozzle.
Japanese Unexamined Patent Publication No. 53-53525 discloses a similar method of continuously manufacturing a metal strip, which comprises providing a slotted nozzle having a rectangular opening in a position opposite to a moving cooling plate with a distance of from 0.03 to 1 mm in between. Molten metal is fed onto the cooling plate which moves at a prescribed speed, within a range of 100 to 2,000 m/minute, in a state in which an orifice of the nozzle is arranged substantially vertical to the surface of the cooling plate. The molten metal is rapidly cooled by bringing the molten metal into thermal contact to cause solidification. By this method, the shape of the rectangular opening of the nozzle can be made longer in a direction at right angles to the moving direction of the cooling plate, and hence, the width of the thin strip can be made larger, because, in theory, there is no restriction on the width of the thin strip.
However, in actual practice, as the width of the thin strip to be manufactured becomes larger, i.e., as the length of the rectangular opening becomes longer, deformation of the nozzle opening caused by thermal stress or ejecting pressure becomes problematic. It becomes difficult to maintain the parallelism of the nozzle opening gap during casting.
In view of this problem, Japanese Unexamined Patent Publication No. 58-132357 discloses prevention of nozzle deformation by providing a weir in the interior of the nozzle slit. The disclosure suggests that it is possible to manufacture a thin strip with a width of 150 mm, having a uniform thickness in the width direction.
Japanese Unexamined Patent Publication No. 63-220950 and Japanese Unexamined Patent Publication No. 1-170554 disclose a method of manufacturing a thin strip by means of a nozzle having a discontinuous opening in the width direction of the strip, in place of a rectangularly slotted nozzle. By this method, it is possible to manufacture a thin strip having a uniform thickness in the width direction by specifying the shape and arrangement of the opening. In theory, it is possible to increase the width of the thin strip without limitation.
This method poses problems in industrial applications because the nozzle has a complicated shape, which results in high processing costs. Furthermore, a deviation of the thin strip thickness in the width direction is caused by not only the slit deformation but by disturbance of the molten metal flow or non-uniform solidification under the effect of: (1) thermal deformation of the roll and the nozzle; (2) temperature of the roll, molten metal and the nozzle; (3) the puddle length due to surface tension of the molten metal; and (4) non-uniformity of the air flow produced by the roll rotation in the width direction of the thin strip. Therefore, thickness deviations in the width direction cannot be completely prevented by prevention of slit deformation alone.
Under these circumstances, the present inventors carried out a casting experiment using a nozzle having an opening as shown in FIG. 2, with the goal of achieving uniform thin strip thickness by preventing deformation of the nozzle opening. The resultant thin strip displayed a thickness profile, as shown in FIG. 3, similar to that available when using a conventional nozzle having a rectangular opening. More specifically, the results illustrate the difficulty of achieving uniform thickness in the width direction through only preventing slit deformation. The results also illustrate the necessity of taking comprehensive measures to prevent slit deformation.
In contrast, Japanese Unexamined Patent Publication Nos. 57-103761, 57-103763 and 62-166056 disclose methods and apparatuses for eliminating the deviation of the molten steel flow rate through either varying the slit gap in the longitudinal direction or by longitudinally varying the bore diameter at the opening comprising a row of small holes.
These known techniques only relate to the casting of a thin strip having a maximum width of 20 mm. It is difficult to apply such techniques to the casting of a wide metal thin strip. The known techniques do suggest the possibility of eliminating deviations in the molten steel flow rate by imparting a thickness (or bore diameter) distribution in the width direction of the nozzle. Those disclosures, however, fail to describe a method of controlling the thickness distribution of the thin strip. Therefore, the problem of how to set a longitudinal distribution of the slit gap, when casting a wide metal thin strip with a thickness uniform in the width direction, is left unsolved.
Practical difficulties arise when the thin strip thickness is non-uniform in the width direction. For example, quality problems arise when non-uniform metal strips are utilized as laminates or for coil processing for magnetic materials in transformers. This results in an unstable coiling for transfer during manufacture.